linux-kernel - Re: [PATCH] memcg: remove unneeded preempt

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Date:	Thu, 18 Aug 2011 14:27:51 -0400
From:	Valdis.Kletnieks@...edu
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Greg Thelen <gthelen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: remove unneeded preempt_disable

On Thu, 18 Aug 2011 16:41:53 +0200, Johannes Weiner said:
> On Thu, Aug 18, 2011 at 10:26:58AM -0400, Valdis.Kletnieks@...edu wrote:
> > On Thu, 18 Aug 2011 11:38:00 +0200, Johannes Weiner said:
> > 
> > > Note that on non-x86, these operations themselves actually disable and
> > > reenable preemption each time, so you trade a pair of add and sub on
> > > x86
> > > 
> > > -	preempt_disable()
> > > 	__this_cpu_xxx()
> > > 	__this_cpu_yyy()
> > > -	preempt_enable()
> > > 
> > > with
> > > 
> > > 	preempt_disable()
> > > 	__this_cpu_xxx()
> > > +	preempt_enable()
> > > +	preempt_disable()
> > > 	__this_cpu_yyy()
> > > 	preempt_enable()
> > > 
> > > everywhere else.
> > 
> > That would be an unexpected race condition on non-x86, if you expected _xxx and
> > _yyy to be done together without a preempt between them. Would take mere
> > mortals forever to figure that one out. :)
> 
> That should be fine, we don't require the two counters to be perfectly
> coherent with respect to each other, which is the justification for
> this optimization in the first place.

I meant the general case - when reviewing code, I wouldn't expect 2 lines of code
wrapped in preempt disable/enable to have a preempt window in the middle. ;)

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